Optical time domain reflectometers (OTDR) as optical pulse testers emit optical pulse to be incident on a measured optical fiber, detect reflected light returned from the measured optical fiber according to the incidence of the optical pulse, so as to detect a fault point position of the measured optical fiber based on a detection result of the reflected light, and measure a transmission loss property and a connection loss property of the measured optical fiber.
FIG. 9 is a block diagram illustrating a constitution of the optical pulse tester disclosed in the following Patent Document 1 as one example of a conventional optical time domain reflectometer (OTDR) as an optical pulse tester which is conventionally known in general.
An optical pulse tester (OTDR) 49 shown in FIG. 9 is composed of a timing generating section 52, a driving circuit 53, a light source 54, an optical directional coupler 55, a light receiver 56, an amplifying section 57, a signal-averaged processing section 58, a logarithmic converting section 59, and a display section 60.
This kind of optical pulse tester (OTDR) 49 generates a pulse current in the driving circuit 53 based on an electric pulse from the timing generating section 52 so as to allow the light source 54 to emit light.
An optical pulse output from the light source 54 passes through the optical directional coupler 55, and is incident on a measured optical fiber 61 to be tested.
Return light such as backscattered light or reflected light from the measured optical fiber 61 is transmitted from the optical directional coupler 55 to the light receiver 56.
The light receiver 56 converts the return light such as the backscattered light or the reflected light from the measured optical fiber 61, which is transmitted from the optical directional coupler 55 to the light receiver 56, into an electric signal.
The electric signal output from the light receiver 56 is amplified by the amplifying section 57.
The signal-averaged processing section 58 converts the analog electric signal amplified by the amplifying section 57 into a digital signal by means of a built-in analog/digital (A/D) converter, and adds the signals a predetermined number of times so as to average them.
An averaged output from the signal-averaged processing section 58 is logarithmically converted by the logarithmic converting section 59, and the logarithmically converted result is displayed normally as a downward-sloping measurement waveform on the display section 60.
In this kind of optical pulse tester (OTDR) 49, basically, two kinds of measuring methods, i.e., averaging measurement and real-time measurement are known.
The averaging measurement is for adding measurement waveforms repeatedly a predetermined number of times for a relatively long time from several seconds to several tens of seconds and averaging them so as to acquire a measurement waveform with excellent signal-to-noise ratio (S/N ratio).
In the averaging measurement, like the optical pulse tester (OTDR) 49 disclosed in Patent Document 1, while a gain of the amplifying section 57 and the number of adding times (the average number of times) in the signal-averaged processing section 58 are suitably switched, the measurement is taken on portions of the measurement waveform with unsatisfactory S/N ratio, and only portions of the measurement waveforms with excellent S/N ratio are jointed so that a region with excellent S/N ratio of not less than a predetermined value which can be used for observation in a wide range can be acquired.
In this case, the optical pulse tester (OTDR) 49 shown in FIG. 9 further includes an S/N ratio comparing section 50 and a data memory section 51 in addition to the above constitution.
Data on the measurement waveform portions with excellent S/N ratio of not less than the predetermined value are directly stored in the data memory section 51 based on the comparison result of the S/N ratio comparing section 50.
Then, while the gain of the amplifying section 57 and the number of adding times (the average number of times) in the signal-averaged processing section 58 are suitably switched, the measurement is taken on the measurement waveform portions with an unsatisfactory S/N ratio of not more than the predetermined value based on the comparison result of the S/N ratio comparing section 50. As a result, data of measurement waveform portions with excellent S/N ratio improved to not less than the predetermined value are sequentially stored in the data memory section 51.
As a result, only the data of the measurement waveform portions with excellent S/N ratio of not less than the predetermined value stored in the data memory section 51 are jointed and displayed on the display section 60, so that an area with excellent S/N ratio of not less than the predetermined value which can be used for observation can be acquired over a wide range.
This can be regarded also as that since the gain of the amplifying section 57 cannot be continuously changed, when a specified combination of hard setting including the gain and a frequency property of the amplifying section 57 and the number of adding times (the average number of times) in the signal-averaged processing section 58 is normally expressed by an attenuator value, plural sets of the attenuator values are expressed by attenuation quantity in units of dB, and thus the attenuator value which becomes optimum according to the S/N ratio of the measurement waveforms is set suitably.
Therefore, it can be regarded also as that the amplifying section 57 and the signal-averaged processing section 58 compose an attenuator which is equivalent to an optical signal before converted into an electric signal.
In the case where the averaging measurement is taken, in the equivalent attenuator composed of the amplifying section 57 and the signal-averaged processing section 58, while the specified attenuator value obtained by the plural combinations of the hard setting including the gains and the frequency property of the amplifying section 57 and the average number of adding times in the signal-averaged processing section 58 is suitably changed according to the S/N ratio of the measurement waveform, the measurement is taken. As a result, solely the measurement waveform portions with an excellent S/N ratio of not less than the predetermined value can be jointed.
Contrary to the averaging measurement, in which a measurement is taken while the attenuator value is suitably changed, real-time measurement is suitable for adding measurement waveforms for a comparatively short time from 0.1 sec to about 1 sec and averaging them with the attenuator value being fixed, and the measurement result is sequentially updated to be displayed so as to observe a condition and a change of the optical fiber at that time.
For this reason, real-time measurement is widely used for applications requiring speed, as in the case where while an optical fiber is being connected by connector connection and fusion, a good/bad condition of the connection of the optical fiber needs to be checked while laying the optical fiber.
The backscattered light which returns from the measured optical fiber 61 to be measured is caused by Rayleight scattering generated in the optical fiber 61.
The level of the backscattered light becomes lower than the level of the incident pulse light by about 50 dB when the measured optical fiber 61 is a normal single-mode optical fiber and the width of an optical pulse incident on the measured optical fiber is 1×10−6 seconds.
Therefore, in order to process such a very small signal, the optical pulse tester (OTDR) 49 needs to improve the S/N ratio by using digital averaging for repeatedly taking measurements a predetermined number of times, and adding and averaging the measurement results.
The signal-averaged processing section 58 of the optical pulse tester (OTDR) 49 shown in FIG. 9 uses such digital averaging so as to improve the S/N ratio.
In the case where digital averaging is used, when a quantization bit of the A/D converter incorporated in the signal-averaged processing section 58 is 8, a relationship shown in FIG. 10 holds between the number of averaging times and the S/N ratio.
In FIG. 10, for example, when the number of averaging times is 100 and the S/N ratio is −30 dB, the S/N ratio becomes 10 dB at the time when the average is 102 times, and this means that the S/N ratio is improved by 20 dB.
A technique for improving the S/N ratio using such digital averaging is described also in the following Non-Patent Document 1.
FIG. 11 is a diagram illustrating a measurement waveform when reflection attenuation whose level difference in the measurement waveform is large is measured by averaging measurement using the conventional optical pulse tester (OTDR) 49 shown in FIG. 9.
That is to say, as shown in FIG. 11, in the case where the reflection attenuation whose level difference in the measurement waveform is large is measured by averaging measurement, more time is taken so that the number of times for adding (the average number of times) the measurement waveform can be sufficiently increased. For this reason, the waveform level just before Fresnel reflection on the position of a marker 1 and the level of an apex of the Fresnel reflection on the position of a marker 2 can be observed simultaneously, and thus the reflection attenuation can be measured based on a difference therebetween.
The measurement of the reflection attenuation is taken when an operator, other than the operator of the optical pulse tester (OTDR) 49, fuses end surfaces of the optical fibers to be connected at a laying site of the optical fibers in a remote place distant from the measurement end by 20 Km or more.
Since it takes a certain amount of time to measure the reflection attenuation in the averaging measurement, when it is clear that the measurement result of the reflection attenuation deviates from an acceptable range, frequently the fusing operator performs the fusing operation for a subsequent optical fiber, and thus the measurement and the fusion cannot take place simultaneously.
On the other hand, in real-time measurement using an optical pulse tester (OTDR) 49, since it does not take much time to measure the reflection attenuation, the problem that the measurement and the fusion cannot take place simultaneously can be avoided.
In real-time measurement, the number of times for adding (the average number of times) the measurement waveforms is smaller than that in averaging measurement, and the measurement is taken with the attenuator value fixed. As a result, the area which is suitable for the observation of the waveforms with excellent S/N ratio in the acquired measurement waveforms becomes narrow.
For this reason, in the conventional optical pulse tester (OTDR) 49 shown in FIG. 9, in the real-time measurement, in the case where the waveform level to be measured greatly fluctuates in the acquired measurement waveform and another distance condition is tried to be observed, the waveform level of the distance to which the operator pays attention easily deviates from a suitable measurement area.
Further, in the conventional optical pulse tester (OTDR) 49 shown in FIG. 9, in the real-time measurement, in the case where a loss value between two points in the acquired measurement waveform is desired to be known and the reflection attenuation calculated based on the loss value between the two points is desired to be measured, the attenuator value cannot be set so that the waveform levels of the two points are simultaneously included in the area suitable for the measurement at the time when the waveform level difference between the two points is large.
Therefore, in the conventional optical pulse tester (OTDR) 49 shown in FIG. 9, it is very difficult to take satisfactory real-time measurement including the measurement of the reflection attenuation under such a circumstance.
FIGS. 12 and 13 are diagrams illustrating the measurement waveforms when the real-time measurement is taken by using the conventional optical pulse tester (OTDR) 49 shown in FIG. 9.
As shown in FIG. 12, in the case where the reflection attenuation in which the level difference is large is measured by the real-time measurement, when the attenuator values are selected so that the waveform level of an apex of the Fresnel reflection on the position of the marker 2 can be measured, the S/N ratio of the waveform level just before the Fresnel reflection on the position of the marker 1 is deteriorated and the waveform cannot be seen.
As shown in FIG. 13, the reflection attenuation where the level difference is large is measured by the real-time measurement. In this case, when the attenuator values are selected according to the waveform level just before the Fresnel reflection on the position of the marker 1, the waveform level of the apex of the Fresnel reflection on the position of the marker 2 is saturated so that accurate measurement cannot be taken.
Therefore, in the real-time measurement shown in FIGS. 12 and 13, in any case, it is difficult to accurately measure the reflection attenuation.
Further, the real-time measurement is taken for applications requiring the readiness with respect to the change in the measured optical fiber state.
In the case where the state of the measured optical fiber which is supposed to be measured and the measured optical fiber which is supposed to be measured is changed into another optical fiber with a certain optical fiber being measured, the attenuator values should be changed manually by the operator in order to adjust a measurement range.
On the other hand, in the real-time measurement, it is desired that the waveform on a desired position can be observed with an excellent S/N ratio of not less than the predetermined value merely by moving the marker as measurement position specifying means to be displayed on the display section 60 to a target position.
For example, in the real-time measurement, the marker (not shown) is moved to a portion of a measurement waveform with an unsatisfactory S/N ratio of not more than the predetermined value on the display section 60, and the attenuator values are changed into an optimum value automatically according to the S/N ratio on the marker position so that the measurement is taken. As a result, it is desired that the S/N ratio is improved on the marker position and a measurement waveform with excellent S/N ratio of not less than the predetermined value can be observed automatically.
In the conventional optical pulse tester (OTDR) 49 shown in FIG. 9, in the real-time measurement, it is extremely difficult to comply with such kinds of desires in view of the many problems described above.    Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 04-158237    Non-Patent Document 1: “Backscattering Measurement and Fault Location in Optical Fibers” Kenji OKADA et al. THE TRANSACTION OF THE IECE JAPAN. VOL. E 63. NO. 2. ABSTRACTS FEBRUARY 1980 pp 145-146